See if this help as they talk about incorrect url configuration.

https://support.mozilla.org/en-US/questions/1176578

You say non-existent -- from Firefox's perspective -- but they do exist as local DNS entries on your system? And I assume not in a hosts file?

By default, Firefox prefers DNS over HTTPS, but can fall back to a local lookup. If your DNS service is dishonest about not having the address, it may divert you to a "search" page of paid links.

As a starting point, you could disable DNS over HTTPS and see whether that makes any difference in Firefox's ability to resolve your local addresses: Firefox DNS-over-HTTPS.

You say non-existent -- from Firefox's perspective -- but they do exist as local DNS entries on your system? And I assume not in a hosts file? By default, Firefox prefers DNS over HTTPS, but can fall back to a local lookup. If your DNS service is dishonest about not having the address, it may divert you to a "search" page of paid links. As a starting point, you could disable DNS over HTTPS and see whether that makes any difference in Firefox's ability to resolve your local addresses: Firefox DNS-over-HTTPS.

Thanks for the reply.

Actually, domain I am using exist in hosts file. Domain is working and some pages are working, but there is a few links that I just can't open. I type on purpose wrong page at domain, firefox force me to search, and don't open the link as expected.

Thanks for the link https://support.mozilla.org/en-US/kb/firefox-dns-over-https

Firefox DNS-over-HTTPS is not what I used, but I used "Excluding specific domains" part and excluded domain I am using localy, now everything is working and I can finally use Firefox to continue working on my app.

Thank you a lot!

Thank you for reporting back.